As one embodiment of the point-to-multipoint optical communication system, there has been discussed extension of a transmission distance by providing a regenerative repeater and a wavelength converter between a station apparatus and a subscriber apparatus. There has been further discussed improvement in efficiency of sharing an optical fiber transmission line by performing multiplexing in a plurality of point-to-multipoint optical communication systems through a wavelength division multiplexing technology (see, for example, Patent Document 1).
In the conventional configuration, on a subscriber apparatus side, one wavelength converter and a multiplexer (MUX)/demultiplexer (DEMUX) for multiplexing and demultiplexing wavelengths are provided between the station apparatus and the plurality of subscriber apparatuses. Further, on the station apparatus side, a MUX/DEMUX for multiplexing and demultiplexing wavelengths is provided.
In such a system configuration as described above, upstream optical signals each having a wavelength of 1.3 μm, which have been transmitted from the plurality of subscriber apparatuses, are regenerated and repeated by the wavelength converter provided between the station apparatus and the plurality of subscriber apparatuses.
More specifically, upstream optical signals are regenerated and repeated through the following series of processing. First, upstream optical signals are opto-electrically converted into electric signals by a light receiving element. Subsequently, the electric signals obtained through the opto-electric conversion are electrically regenerated by a reception circuit having a 2R function or a 3R function. Further, the signals thus electrically regenerated are electro-optically converted into optical signals again by a driver circuit and a light emitting element.
In addition, at the same time when the regenerative repeating is performed, the wavelength of 1.3 μm is converted into a wavelength λi. Then, wavelength division multiplexing is performed by the MUX/DEMUX on the subscriber side, to thereby realize wavelength division multiplexing transmission employing the wavelength division multiplexing (WDM) technology in an extension section.
Further, on the station apparatus side, wavelength division demultiplexing is performed by the MUX/DEMUX provided on the station apparatus side, and therefore the signals each having the wavelength λi are extracted, which are received by an i-th station apparatus.
Conversely, for downstream optical signals each having the wavelength λi, which have been transmitted from the i-th station apparatus, wavelength division multiplexing is performed by the MUX/DEMUX provided on the station apparatus side, to thereby realize the wavelength division multiplexing transmission employing the wavelength division multiplexing technology in the extension section.
Further, similarly to the case of the upstream optical signals, the MUX/DEMUX on the subscriber side, which is provided between the station apparatus and the plurality of subscriber apparatuses, demultiplexes the optical signals, on which the wavelength division multiplexing has been performed, to extract the signals each having the i-th wavelength λi. In addition, the wavelength converter performs opto-electric conversion, regenerative repeating, and electro-optic conversion on the extracted optical signals each having the wavelength λi, and at the same time, converts the wavelength λi into the wavelength of 1.3 μm. Finally, the plurality of subscriber apparatuses receive the signals having the wavelength of 1.3 μm obtained through the wavelength conversion, respectively.
Further, in the configuration of the conventional wavelength converter, for both the upstream signals and the downstream optical signals, the wavelength of 1.3 μm is used in an access section, while the wavelength λi is used in the extension section. Therefore, a 3-dB coupler is adopted for multiplexing and demultiplexing the upstream signals and the downstream signals.
Further, the upstream signals and the downstream signals are regenerated and repeated in a separate manner in the wavelength converter. Hence, the upstream signals and the downstream signals do not affect each other basically.
As described above, adopting the wavelength converter and the MUX/DEMUX, which are described in Patent Document 1, enables extension of the transmission distance in the extension section corresponding to the section between the station apparatus and the wavelength converter, in addition to the conventional access section corresponding to the section between the station apparatus and the subscriber apparatus.
Patent Document 1: JP 2002-261697 A